In an attempt to increase the efficiencies and performance of contemporary jet engines, and gas turbine engines generally, engineers have progressively pushed the engine environment to more extreme operating conditions. The harsh operating conditions of high temperature and pressure that are now frequently specified place increased demands on engine components and materials. Indeed the gradual change in engine design has come about in part due to the increased strength and durability of new materials that can withstand the operating conditions present in the modern gas turbine engine.
The turbine blade is one engine component that directly experiences severe engine conditions. Turbine blades are thus designed and manufactured to perform under repeated cycles of high stress and high temperature. An economic consequence of such a design criteria is that currently used turbine blades can be quite expensive. It is thus highly desirable to maintain turbine blades in service for as long as possible. It is correspondingly desirable to manufacture and finish turbine blades so as to withstand the corrosive and erosive forces that will attack turbine blade materials.
Turbine blades used in modern gas turbine engines are frequently castings from a class of materials known as superalloys. The superalloys include alloys with high levels of cobalt and/or nickel. Therefore, nickel and cobalt based superalloys are thus preferred materials for the construction of turbine components, including blades and vanes. The high strength nickel-based superalloys are noted as precipitation hardening alloys. Nickel, alloyed with elements such as aluminum and titanium, develops high strength characteristics that are sustainable at high temperatures. The strength arises predominantly through the presence of a gamma prime (γ′) phase which is an intermetallic compound formed between nickel and Al or Ti or both in the material. One characteristic of the advanced nickel-based superalloys is the high degree of gamma prime (60% or more volume fraction) in cast materials.
In the cast form, turbine blades made from superalloys display many desirable physical properties and mechanical properties including high strength at elevated temperatures. Advantageously, the strength displayed by this class of materials remains present even under arduous conditions, such as high temperature and high pressure. Disadvantageously, the superalloys generally can be subject to corrosion and oxidation at the high temperature operating regime. Sulfidation can also occur in those turbine blades subject to hot exhaust gases.
Thus, it has become known to provide coatings or protective layers on engine components, such as turbine blades, that are subject to corrosion, erosion or sulfidation. Many components in the advanced turbine engine hot section, in addition to turbine blades, also require protective coatings for resistance to oxidation, sulfidation, and corrosion. Chromium, aluminum, and other metallic coatings can be used to provide protective layers that are more resistant to corrosion and/or oxidation than is the underlying substrate material. In the case of superalloys, materials such as platinum, aluminum, and chromium can be used to provide protective coatings.
Various coating types and various coating deposition systems have been developed. In extremely high temperature applications, a Thermal Barrier Coating (TBC) may be needed to provide the required heat resistance. A TBC typically is composed of ceramic materials such as zirconia, (ZrO2), yttria (Y2O3), magnesia (MgO), or other oxides. Yttria Stabilized Zirconia (YSZ) is a widely used TBC. A TBC is often used in conjunction with an underlying metallic bond coat.
Metallic coating systems for use as Environmental Barrier Coatings (EBC) and as TBC bond coatings for gas turbine engine components include diffusion-based coatings and overlay coatings. A diffusion coating may include aluminides and platinum aluminides. Pack cementation used for diffusion coating formation is a common method whereby metallic vapors of the desired coating are carried to the surface of a target and diffused thereon. These diffusion coatings are somewhat limited by the difficulty of codepositing other metals along with aluminum onto the substrate surface.
A common overlay coating used for HPT components is known as MCrAlY. In the conventional formulation of MCrAlY, M represents one of the metals nickel, cobalt, or iron, or combinations thereof. In the designation MCrAlY, Cr, Al, and Y are the chemical symbols for chromium, aluminum, and yttrium. Some conventional MCrAlY formulations are discussed in the following U.S. Pat. Nos. 4,532,191; 4,246,323; and 3,676,085. Families of MCrAlY compositions are built around the nickel, cobalt, or iron constituents. Thus the literature speaks of NiCrAlY, NiCoCrAlY, CoCrAlY, CoNiCrAlY, and so on.
The family of MCrAlY coatings offer an alternative to the diffusion-based coatings in that elements beyond aluminum and platinum are included in the coating, which brings an attendant improvement in corrosion and/or oxidation resistance. However, the MCrAlY coatings are not diffusion coatings and result in a distinct layer from the substrate as the coating; hence they are often referred to as overlay coatings. Many high temperature overlay coatings are produced by processes such as PVD, EBPVD, HVOF and LPPS.
The prior art methods of providing environmental and bond coatings have experienced limitations and drawbacks. For example it is difficult with spray techniques to obtain a homogenous, high-quality and dense coating. Chemical vapor deposition methods suffer from slow deposition rates and a difficulty in accommodating large components. The physical vapor deposition process faces difficulty in deposition rates and in efficiently applying cost effective coatings. And, diffusion coatings are limited in their ability to efficiently provide multiple elements in a single diffusion step. Thus there is an ongoing need for improved methods of applying coatings.
Additionally, coatings can be improved in their effectiveness over a wide spectrum of operating temperature regimes and in response to a range of environmental stresses. For example, platinum aluminide coatings (including the class of platinum modified nickel aluminides) are utilized as straightforward oxidation resistance coatings and as “bond coats” for thermal barrier coating (TBC) applications. But, currently used platinum aluminides do not utilize the beneficial effects of chromium and active elemental additions. Furthermore, there is a need to incorporate chromium, which improves the Type II sulfidation resistance of the coating, to supplement the good Type I sulfidation resistance and oxidation resistance exhibited by the platinum aluminide coatings.
Hence there is a need for an improved method to apply a protective coating. There is a need for an improved coating method that can be easily and effectively applied. Further the composition of the coating should include active elements, as well as chromium in the platinum aluminide coating, in order to provide effective oxidation, corrosion, and sulfidation resistance over a broad temperature range. Such coatings are needed to provide wide spectrum EBC protection for Type I & II sulfidation and high temperature oxidation resistance. Such coatings are needed as well for improved bond coats used for TBC applications. There is additionally a need to provide processing steps to offer duplex coating microstructures which are different for different sections of HPT blades. For instance, gas path surfaces above the platform may require certain coating characteristics whereas the shank portion between the dovetail and blade platform sections may require a different coating microstructure. The present invention addresses one or more of these needs.